The present disclosure generally relates to drilling operations and, more specifically, to methods for softening a water source during formulation of drilling fluids.
Treatment fluids can be used in a variety of subterranean treatment operations. Such treatment operations can include, without limitation, drilling operations, stimulation operations, production operations, remediation operations, sand control treatments, and the like. As used herein, the terms “treat,” “treatment,” “treating,” and grammatical equivalents thereof will refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. Use of these terms does not imply any particular action by the treatment fluid or a component thereof, unless otherwise specified herein. More specific examples of illustrative treatment operations can include, for example, drilling operations, fracturing operations, gravel packing operations, acidizing operations, scale dissolution and removal operations, sand control operations, consolidation operations, and the like.
A drilling fluid is usually used during drilling operations, for example, to cool the drill bit, to control pressure within the wellbore, and to suspend and transport drill cuttings from the wellbore to the earth's surface. It is also usually desirable to control loss of the drilling fluid from the wellbore into porous features of the subterranean matrix. Although fluid loss control is desirable during drilling operations for a number of reasons, weakening of the subterranean matrix due to excessive fluid incursion is often an overriding concern. Fluid loss into the subterranean matrix can usually be lessened by forming a filter cake within the wellbore. In most instances, the drilling fluid may be formulated with materials that promote formation of a filter cake upon initial spurt loss of the drilling fluid to the subterranean matrix. Depending upon the type of wellbore being drilled, the filter cake may be allowed to remain intact following drilling or undergo active or passive degradation. For example, in oil-bearing wells, the filter cake may be removed to allow production to commence, whereas in water wells and mineral exploration wells, the filter cake may be allowed to remain intact, if desired.
In order to provide drilling fluids with suitable rheological properties for conveying drill cuttings and other particulate matter within a wellbore, viscosifying agents are often employed as a component of the drilling fluids. Both clay-based and polymer-based viscosifying agents may be used for this purpose. Various metal ions located within a water source used to formulate a drilling fluid may compromise the ability of both types of viscosifying agents to function properly. Calcium ions and other alkaline earth metal ions, for example, can be particularly problematic when present in a water source, since they can result in clay flocculation and/or polymer precipitation when formulation of a drilling fluid takes place. Alkali metal ions, such as sodium ions, may be considered to be essentially non-problematic. As used herein, the term “hard water” will refer to a water source containing one or more alkaline earth metal ions. Other metal ions, such as transition metal ions, may be similarly problematic and present in a hard water source. The term “hardness” will refer to the quantity of alkaline earth and/or transition metal ions that are present in a given water source or, more generally, the extent to which the water source is “hard.”
Despite the potential difficulties associated with using hard water sources for formulating drilling fluids, it can still be desirable to use such water sources in deference to deionized water sources, which may be much more costly or not readily available for drilling fluid formulation. To make a hard water source suitable for formulation into a drilling fluid, the water source may be pre-conditioned through various softening processes. As used herein, the term “softening” and grammatical variants thereof will refer to lessening of the hardness of a water source. Softening of a water source is usually conducted prior to combining a viscosifying agent with the water source to form a drilling fluid. Once the viscosifying agent has contacted the metal ions in a hard water source, there is usually no way to reverse the degradation of the viscosity properties that may result.
Soda ash (sodium carbonate) is a widely used softening agent for hard water sources that are being formulated into drilling fluids. This softening agent is inexpensive, environmentally friendly, and may provide drilling fluids having good rheological and fluid loss control performance with both clay-based and polymer-based drilling fluids. However, the commercial use of soda ash is becoming increasingly regulated due to its utilization in the manufacture of illegal drugs, and this softening agent is no longer a viable option for large-scale use in some countries, particularly in Latin America and South America. Alternative softening agents that may be used in the formulation of drilling fluids include various blend materials containing sodium hydroxide (e.g., sodium bicarbonate/sodium hydroxide blend). However, such blend materials often provide inferior rheological and fluid loss control performance compared to soda ash, and the highly caustic nature of sodium hydroxide can also be problematic.